This invention relates to a magnetic field generator for an MRI, a method for assembling the same, and a method for assembling a magnet unit for the same. More specifically, this invention relates to a magnetic field generator for MRI incorporating permanent magnets, a method for assembling the same, and a method for assembling a magnet unit for the same. It will be appreciated, however, that the-invention is also amenable to other like applications for complex assembly of components exhibiting large interaction forces between the members to be assembled.
A magnetic field generator for MRI uses permanent magnets. The magnets used in such an apparatus are often formulated from a plurality of magnet blocks. It is very difficult to place material blocks first and then magnetize each block. Therefore, in actual manufacturing, the blocks are fabricated and then magnetized. The magnetized blocks are then arranged on a plate yoke so that each of the magnet blocks has a same magnetic pole facing upward. Such arrangement on a plate yoke is difficult due to the interaction of the large magnetic forces between each of the magnet blocks.
Conventionally, when placing the magnet blocks on the plate yoke, a surface of the plate yoke or magnet block is first applied with adhesive, and then magnet blocks are bonded or attached to the surface, as disclosed in the Japanese Patent No. 2,699,250 for example. According to such a bonding method, upper surfaces of respective magnet blocks bonded to the plate yoke surface are not flush with each other, making an uneven surface. A magnetic field generator incorporating the permanent magnets made of such magnet blocks is apt to produce non-uniform magnetic field between a pair of piece poles opposed to each other. Further, pole pieces for correcting the non-uniformity of the magnetic field may be tilted to produce non-uniformity in the magnetic field. Generally, after a step of mounting a pair of permanent magnets to oppose each other, a step of adjustment for uniformly distributing the magnetic field is indispensable. However, if the magnet blocks are mounted according to the above method, the non-uniformity of the magnetic field is so large that the adjustment becomes very time consuming.
Further, according to the above method of bonding the magnet blocks, the magnet blocks each exhibiting very large magnetic forces is placed from above, onto the upper surface of the plate yoke, making extremely difficult to fit each of the magnet blocks snugly to adjacent magnet blocks. More specifically, when mounting, each magnet block is held with a face of predetermined magnetic pole facing upward. When the magnet block is brought above the other magnet block, which is already fixed onto the plate yoke, a pulling force is generated between the two. Further, when the two magnet blocks are brought in adjacency, a repelling force is generated between the two. Since the magnet block to be placed is under such intense forces, the magnet block must be firmly held for safety while being transported. For a conventional holding mechanism, it is very difficult to fit the magnet block snugly to the place of bonding efficiently against these strong forces.
The pair of magnet units thus assembled as above is then opposed to each other so the permanent magnets are faced at one another at a predetermined distance. This is achieved by first assembling one magnet unit, then connecting one or more posts or a column yoke to the magnet unit, and finally connecting the other magnet unit to the post(s).
The post(s) magnetically connect the pair of magnet units, and therefore must be made of a magnetic material. Thus, when the post is connected to the magnet unit, the post is brought under the pulling force from the magnet unit. This large force makes it difficult to connect the two plate yokes with the high accuracy. Likewise, when the second magnetic unit is connected to the post already connected to the first magnet unit, it is also difficult to connect the two at a high accuracy.
Another method to assemble a magnetic field generator is disclosed by EP0978727A2. In this patent, a non-magnetic fixed projection is place at the center of the yoke, with two orthogonal guide rails. The magnetic blocks are then slid into place and bonded to each other along the non-magnetic fixed projection and guide rails. This approach while adequate for its intended purposes is still cumbersome and requires additional special tooling. What is desired is a method for assembling the magnetic field generator to desired tolerances with a minimum of specialized tooling and assembly steps.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a magnetic field generator and method for assembling the same, for a magnetic resonance imaging system, the method comprising: establishing a layout for a permanent magnet of a magnet assembly comprising a pole piece, a ferromagnetic plate yoke and a permanent magnet. The method also includes populating the layout with a plurality of mock-up sticks and block retainers to form a mock layout for the permanent magnet; and installing a magnet block in place of at least one mock-up stick of the plurality of mock-up sticks. Installing the magnet blocks includes pushing the magnet block along a selected slot formed by the displacement of at least one mock-up stick of the plurality of mock-up sticks.
The method of may also include repeating the abovementioned installing until each mock-up stick of the plurality of mock-up sticks is replaced by a plurality of magnet blocks. Additionally, the method may also include installing a pole piece positioning tool to facilitate location and placement of the pole piece on said magnet assembly.
Also disclosed herein is a magnetic field generator for a magnetic resonance imaging system. The magnet field generator comprises a layout for a permanent magnet of a magnet assembly comprising a pole piece, a ferromagnetic plate yoke and a permanent magnet. The magnet field generator also comprises a plurality of mock-up sticks and block retainers populating the layout to form a mock layout for the permanent magnet; and a magnet block installed in place of at least one mock-up stick of the plurality of mock-up sticks. The installation of the magnet block comprises pushing the magnet block along a selected slot formed by the displacement of the at least one mock-up stick of the plurality of mock-up sticks.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.